1. Field of the Invention
The invention is directed to optical communication systems in general and, more particularly, to bidirectional optical communication systems that transport wavelength division multiplexed optical signals in opposite directions over the same bidirectional waveguiding medium and which can selectively add or drop one or more arbitrary optical channels at any point along the bidirectional waveguide.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WDM networks, can be found in Ramaswami et al., Optical Networks: A Practical Perspective (Morgan Kaufman, .COPYRGT. 1998), the disclosure of which is incorporated herein by reference. Typically, wavelength division multiplexed optical communication systems have been designed and deployed in the long-haul, interexchange carrier realm. In these long-haul optical systems, a wavelength division multiplexed optical communication signal comprising plural optical channels at different wavelengths travels in a single direction on a single fiber (unidirectional transmission). Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent, occurring at widely-spaced add-drop nodes.
Many designs have been proposed for add-drop multiplexing in unidirectional wavelength division multiplexed optical communication systems. In U.S. Pat. No. 6,061,484, a prior-art single channel unidirectional add-drop multiplexer is depicted. The basic configuration consists of two three-port optical circulators with an in-fiber Bragg grating disposed in a fiber connecting the circulators. A unidirectional WDM signal enters the first circulator; a channel to be dropped is reflected by the grating to a drop port while the remaining channels of the WDM signal pass through to the second circulator. Similarly, a channel to be added enters the second circulator, is output to the same grating and reflected back towards the second circulator. In this way, a channel pair having the same channel wavelength is add-dropped from the unidirectional WDM optical signal.
Various improvements to this class of unidirectional add-drop multiplexers have been proposed. Some of these improvements include tunable grating filters, switchable gratings, multiple channel dropping and/or adding, etc. Such add-drop multiplexers for unidirectional wavelength division multiplexed optical communication systems are described in U.S. Pat. Nos. 5,479,082, 5,608,825, 5,712,932, 5,726,785, 5,748,349, 5,751,456, 5,778,118, 5,822,095, 5,841,918, 5,926,300, 5,946,430, 6,020,986, 6,038,045, 6,040,932, 6,061,484, 6,067,389, 6,069,719, 6,108,468, 6,122,095, and 6,122,096, the disclosures of which are incorporated by reference herein.
Although the designs for such add-drop multiplexers are suitable for conventional unidirectional (typically long-haul interexchange carrier markets) WDM optical systems, metropolitan (local) communications systems require extensive routing and switching of traffic among various nodes positioned within interconnecting optical fiber rings. Consequently, smaller metropolitan markets require considerably more extensive add-drop multiplexing in order to successfully implement wavelength division multiplexing in their short-range systems. Further, in order to maximize the effectiveness of wavelength division multiplexing in these local areas, it would be useful to implement bidirectional WDM optical systems, e.g., to enhance network design flexibility. In a bidirectional WDM system counter-propagating WDM optical signals, each of which carry a number of optical channels, are carried on the same waveguiding medium, such as a single optical fiber. Implementation of a bidirectional system requires several considerations not present in the conventional unidirectional optical systems. Amplification of a bidirectional system requires specially-designed optical fiber amplifiers or convoluted simultaneous routing of the bidirectional WDM signals in a single direction through a unidirectional amplifier. Various configurations for bidirectional amplifiers are depicted in U.S. Pat. Nos. 5,604,627, 5,633,741, 5,742,416, 5,812,306, 5,887,091, 5,995,259, 6,081,368, and 6,101,016, the disclosures of which are incorporated by reference.
In addition to the increased difficulty in amplifying bidirectional WDM optical signals, add-drop multiplexing in the bidirectional optical environment becomes considerably more complex. Further, because frequent add-drop multiplexing is required in local metropolitan systems, a bidirectional add-drop multiplexer must be cost-effective and robust, i.e., capable of repeated operation with little chance of failure (e.g., due to moving parts such as switches, movable reflectors, etc.). Several bidirectional multiplexing designs have been proposed; however, these designs are predominantly directed to the issue of transmitting to or receiving from the optical transmission fiber at the beginning or end point of an optical system (e.g., the system end nodes). In U.S. Pat. No. 5,909,295, optical circulators are used to separate the counter-propagating optical signals that are further filtered down to individual channel wavelengths. In many embodiments, expensive four-port (or higher) optical circulators must be used. Although optical channels are separated, there is no teaching or suggestion of signal recombination such that a bidirectional optical signal continues to propagate along a bidirectional transmission waveguide.
In U.S. Pat. No. 5,748,350, optical circulators and gratings are again used to separate or combine various optical channels. Although the device itself is "bidirectional" in the sense that wavelengths being reflected by a Bragg grating into a circulator propagate in a direction opposite to signal being transmitted through the Bragg gratings, the optical signals placed onto output fibers create a unidirectional WDM signal propagating in a single direction.
Thus, there is a need in the art for a bidirectional add-drop multiplexer for use in bidirectional optical communication systems. Such a bidirectional multiplexer would permit effective implementation of bidirectional wavelength division multiplexing in local, metropolitan markets requiring high volumes of signal re-routing.